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Hao Sun, Barry B. Lee; The origin of the chromatic response of magnocellular ganglion cells. Journal of Vision 2004;4(11):18. doi: 10.1167/4.11.18.
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© ARVO (1962-2015); The Authors (2016-present)
The ganglion cells of the primate magnocellular (MC) pathway form the physiological substrate for photometric tasks and show a response minimum to equiluminant stimuli. However, there are residual second-harmonic (2F) responses to such stimuli at medium and high temporal frequencies (Lee et al., 1989) and first-harmonic (1F) chromatic responses at low temporal frequencies (Smith et al., 1992).
The responses of MC cells to modulation in different directions of an L, M-cone space show that the higher harmonic distortions appear as soon as the stimulus vector moves away from the luminance direction, and are marked to cone-isolating stimuli. This rules out a mechanism based on M, L-cone summation non-linearities and suggests the presence of rectified chromatic signals, which might arise from the parvocellular (PC) ganglion cells. Recent studies show that both the 1F and the 2F responses become smaller if small spots are used, suggesting involvement of the surround. However, when annulus stimuli activating the surround alone are used, both effects disappear. Area-summation experiments show that the 2F response has a diameter of ∼1.5 times the center diameter, which is comparable to PC cells' receptive fields (center+surround). In other experiments, the MC cell residual response to equiluminant borders also shows a similar width to the 2F field. Lastly, it is possible to null out the 2F response on a contrast reversal test with red-green chromatic gratings. This suggests a mechanism in which linear spatial summation occurs before a rectifying non-linearity, rather than a subunit structure.
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